Previously known vertically moving continuous conveyor mechanisms for vending machines have relied primarily on roller tracks having offset elliptical guide channels at the top and the bottom of the vertical travel for maintaining the shelves in a substantially horizontal orientation. For example, conveyor mechanisms such as those depicted in U.S. Pat. Nos. 3,297,378 to Krug, et al. and 3,202,265 to Anders rely upon "scissor" type linkage mechanisms and roller channel guides for maintaining horizontal orientation of the conveyor shelves. The scissor type linkage included two-point connection to the chain and a pivotable apex at which a shelf support arm is connected. The guide roller was mounted on the support arm so that it followed an elliptical guide channel. The entire force counter-rotating the shelf support arms and attached shelves to hold them in a horizontal orientation was exerted through the roller as it moves through the channel guides. The forces subjected the roller to wear and binding in the channel guides. Further, the linkage was subject to wear at each of the pivot points such that the likelihood of binding was accelerated and reliability was reduced.
Another device operating on a principle similar to the "scissor" linkage was disclosed in U.S. Pat. No. 4,942,290 issued to Frercking. Again, the horizontal position of the shelf was maintained through counter-rotating force exerted through a roller extending from a corner of the horizontal shelf. The roller moved in an offset or elliptical guide channel. At each 180.degree. transition the roller is forced to follow an elliptical or arc path so that the shelf is rotated 180.degree. in the opposite direction from the direction that the chain moves around the top and the bottom sprockets. Again, the entire rotational force placed on the shelf was imparted through the roller acting in the channel, which force eventually caused wear of mechanical parts and resulted in binding and accelerated failure.
In other conveyor mechanisms outside of the vending machine art, such as that disclosed in U.S. Pat. No. 3,902,590 issued to Raynor, et al., an endless chain vertical conveyor system with top and bottom 180.degree. transition sprockets for the endless chain is disclosed. A platform which was fastened to the chain through a pivot connection was maintained in a horizontal orientation with a continuously operating planetary gear system. This type of planetary gearing system does not suggest itself for vending machine applications. Further, it has the disadvantage of continuous relative rotation between the sprocket axle and the sun gear, as well as a continuously rotating idler gear. Further, because of chain wear and chain stretch and because of the continuous rotation of the idler gear, relative to a non-rotating shelf gear, this arrangement is likely to result in gear clashing and potentially catastrophic binding and resulting malfunctions or mechanical difficulties. Such a device was not acceptable for the relatively low power application in a vending machine.
Another planetary gearing mechanism for maintaining a conveyor tray in the same angular orientation around transfer points is disclosed in U.S. Pat. No. 2,965,214 to Schlumphf, et al. In particular, a vertical conveyor embodiment was shown in which a sun gear was held in a non-rotating position coaxial with the chain sprocket, which sprocket rotated as the chain and tray moved therearound. The idler gear moved around the sun gear and rotated on an axle mounted on the sprocket. A tray or a shelf was affixed to a gear which engaged the idler gear at the beginning of each 180.degree. transition and disengaged it at the end of each 180.degree. transition. Again, as with the Raynor, et al. device, the binding and gear clashing which was a likely result, does not suggest this mechanism for use in a vending machine. Any chain wear or chain stretch which might be common with roller chains of the type which are commercially available would likely cause improper gear meshing.
Other conveyor systems, as for oven trays and the like, have been used primarily for moving baking trays in a horizontal direction. In these devices, such as the one shown in U.S. Pat. Nos. 2,512,356 to Massiello and 2,493,857 to Cargill, the small vertical rise or fall which occurs at the 180.degree. transition ends of the horizontally moving conveyors was provided through the use of semicircular guide channels in rounded, engaging bosses on either end of the oven trays. The disclosed devices appear to rely primarily upon gravity and sliding during the short vertical fall or rise at the transition ends, rather than from directly applied mechanical force.